Is there any in-game indication that you may summon spirits?

[Kadir]

I know that you can summon spirits only under certain circumstances, you can't summon in multiplayer sessions and usually boss fights when this icon appears:

However, it doesn't seem to be consistent and there are often some encounters where you cannot summon them when it seems like we should be able to. This icon isn't always clear, so I'm looking for something more reliable.

Is there something in-world that indicates that you may summon these spirits?

There's already things like Summoning Pools or Stakes of Marika that make it clear where you can do co-op or resume nearby respectively, surely there's something for summons.

[tahishae]

If you're looking for some kind of physical indicator, the only thing you would be able to summon ashes is by way of a Rebirth Monument

These are the physical indicator that Ashes may be summoned. If you are within range of one of these, then the icon will appear on screen, and you can summon them.

The other thing you can do is keep them on your items, and when they become brighter, it shows that you can summon them.